This invention relates to flexible film laminates and to retortable food pouches fabricated therefrom.
Certain packaging applications, for example, retort packaging, require that the packaging material constitute a good barrier to the passage of oxygen and moisture vapor. Oriented polyacrylonitrile (PAN) film has excellent oxygen barrier properties. Its moisture barrier properties, however, are less than desirable for retort packaging. Furthermore, it is not heat-sealable to itself. Both deficiencies can be overcome by applying to one surface of the PAN a layer of a thermoplastic material having good moisture barrier properties. However, because of the significant chemical dissimilarity between PAN and thermoplastic moisture barrier polymers such as polyolefins, adhesion between the two materials is poor and package integrity suffers. It is necessary, therefore, to consider the imposition of an adhesive system between the PAN and the thermoplastic moisture barrier which will result in acting as a mutually adherent bridge between the two materials.
Prior practice of providing an adhesive bridge between PAN and an effective moisture barrier film has involved the application of an adhesive such as polyurethane or a polyester in a liquid vehicle. By this means, the adhesive is applied to the PAN and subsequently the moisture barrier film is laminated to the PAN via the adhesive. Such procedures have varying degrees of effectiveness but all include the disadvantages of employing costly adhesives and involve solvent coating with its attendant cost and environmental considerations.
In recent years, the food packaging industry has shown intense interest in the concept of pouch-packed foods which, among other advantages, do not require freezing for their preservation and can therefore dispense with costly and energy intensive refrigerated transportation and storage facilities. Much effort has gone into the development of a flexible retortable, or autoclavable, food pouch which can not only withstand the rigors of sterilization and later reheating and provide barrier properties which are sufficient to adequately protect the contents during storage, but which employs materials that are toxicologically safe.
In accordance with U.S. Pat. No. 3,453,173, a polyolefin-polyacrylonitrile laminate, which is said to possess superior heat-seal strength and excellent barrier properties to the transmission of gases and is adaptable for the construction of food containers, is prepared by bringing the polyolefin surface and the polyacrylonitrile surface together and adhering the adjacent surfaces to each other through an adhesive. Either or both surfaces can be pretreated in some manner in an effort to make them more adherent, e.g., by treatment with a gaseous mixture of boron trifluoride as described in British Pat. No. 834,196; by flame treatment or by treatment by corona discharge as described in U.S. Pat. No. 2,632,921. Among the adhesives employed in the manufacture of the laminate is ethylene-vinyl acetate copolymer described in U.S. Pat. No. 2,200,429. Because the conditions of preparation and the adhesives contemplated are not conducive to the formation of primary valence bonding, the interlaminar adhesion in boiling water would not be expected to be strong.
Resort also has been had to metallizing the PAN film in order to employ the metal layer as an effective surface through which adhesion can be accomplished to a water vapor barrier film.